The applicant proposes to test the validity of the hypothesis that both in vitro cytopathicity and monocyte infectability are implicated as important determinants in the pathogenesis of HIV. The first goal of the applicant will be to examine the issue of monocyte infection, specifically, whether CD4-binding and virus integration are mandatory. She will attempt to mutagenize the CD4 binding site on the gp120 envelope (env) gene and the integrase region of the pol (specifies viral reverse transcriptase) gene to determine if mutants defective in these functions could infect different target cells. Next, the investigator proposes to determine if infection of both T cells and monocytes is necessary for induction of CD4+ cell depletion in vivo. From the experiments described above, as well as additional mutagenesis of the env gene, the applicant hopes to obtain viruses that are exclusively lymphotropic or monocyte-tropic. These monotropic viruses will be evaluated by infection of macaques for their capacity to replicate and induce CD4+ cells in vivo. Lastly, the investigator proposes to investigate if in vitro cytopathicity correlates with in vivo virulence. The investigator has previously shown that the C-terminus of the gp41 transmembrane protein of HIV-1 is important for viral infectivity and/or cytopathicity. However, some isolates of HIV-2/SIV (simian immunodeficiency virus) express a truncated form of the transmembrane protein, and the basis for selection of this truncation is not well understood. The investigator proposes to introduce multiple stop codons toward the C-terminus of the HIV-2 genome and to test mutant viruses for infectivity and cytopathicity. Infectious but non-cytopathic viruses will be examined for their pathogenicity in vivo.